1. Field of the Invention
This invention relates to flame ionization detectors for e.g. gas chromatography and more particularly to such detectors of compact size and low fuel requirement.
2. Description Of The Prior Art
One type of detector used in gas chromatography is called a flame ionization detector (FID). The effluent gas (sample) from the analytical column of a gas chromatograph is premixed with hydrogen makeup gas and burned at the top of a flame tip with air (or oxygen) as oxidant. The burning decomposes the organic analytes and ionizes them quantitatively. The resulting ions or electrons are collected by a cylindrical electrode located adjacent the flame tip. Another electrode located below the flame tip provides a bias potential, so that an electrical field develops between the two electrodes. Current flowing between the two electrodes is proportional to the number of ions or electrons produced and collected.
In a flame ionization detector, a diffusion flame is the most efficient way to ionize the analyte (sample). The introduction of air must also be done very smoothly to minimize flicker noise, caused by having a flame burning unsteadily, and typically resulting from an air inlet that does not bring the air to the flame tip smoothly. It is known (see Modern Practice of Gas Chromatagraphy, ed. Robert L. Grob, 1985, p. 244) to introduce the air through a porous diffuser located well below the tip of the jet. Flicker noise is also reduced in the prior art by allowing sufficient vertical distance between the air inlet and the flame tip so as to develop a laminar air flow, however a sufficient vertical distance undesirably results in a physically longer detector.
Conventional flame ionization detectors tend to use a high flow rate of e.g. hydrogen make-up gas (greater than 30 ml./min), thus requiring a correspondingly high air flow rate for the burning process. For portable gas chromatographs, this requires frequent recharge of the hydrogen and air cylinders, which is undesirable. Also, the high flow rate of hydrogen creates a longer and wider flame, requiring a correspondingly physically large collector, hence increasing overall detector size.
A prior art flame ionization detector shown in FIG. 1 includes signal probe 10, igniter probe 12, flame tip assembly 14, spring clip 16 (which is the first electrode), igniter coil 18, insulator 20, and a collector tube (second electrode) 22. (No air diffuser is present in this detector). The flame is created by burning hydrogen plus the analyte in air or oxygen. A bias potential of up to 300 V is maintained between spring clip 16 and collector tube 22. Ions or electrons formed during the combustion process are collected by collector tube 22 and result in the flow of a small electrical current, which is measured by an external circuit. The hydrogen flame by itself produces very few ions or electrons, i.e. very low electrical current, but when an organic compound (e.g. a hydrocarbon) is present in the gas stream in the form of a gas chromatograph peak, large quantities of ions and electrons proportional to the number of organic molecules are created. During combustion of an organic compound, both ions and electrons are formed. Depending on the polarity of the electrodes, either ions or electrons are collected. If the bias electrode (the spring clip of FIG. 1) is negative and the collector tube is positive, electrons will be collected. When the bias electrode is positive and the collector tube is negative or grounded, ions are collected. Ions are usually easier to collect because of their larger size. An elevated current is produced and can be recorded (observed) by an external circuit. A recording of the current versus time reproduces the mass of organic compounds in the gas chromatograph peak.
In addition to requiring relatively high gas flow rates, the prior art detector of FIG. 1 is disadvantageously large, typically having a height H of about 5 to 7 cm. This is because a large distance must be allowed to smooth the inlet air flow in order to reduce the flicker noise caused by the air flow turbulence. The distance h, which is from the air inlet to the top of the flame tip, for this prior art detector is about 2 to 2.5 cms.
Additionally, the flame ionization detector of FIG. 1 is assembled from a number of individually fabricated metal and ceramic components each of which is relatively large. Collector tube 22 is an example. Its height is greater than h, the distance between the air inlet and the top of flame tip. Probes 10 and 12 are also bulky, resulting in an undesirably large horizontal dimension. The large size of this flame ionization detector makes it difficult to use in portable applications.
Flame tip inside diameters used in prior art detectors range from 0.010 to 0.020" (0.254 to 0.508 mm). A larger flame tip requires a larger hydrogen flow to maintain an efficient ionization flame. This produces a larger flame in terms of height and radius and requires a larger collector, thus increasing the size of the detector. To build a small flame ionization detector for portable gas chromatograph, a smaller flow of hydrogen must be used and its use reduces the size of the collector and subsequently the size of the detector in addition to the advantage of having better gas economy and ease of operation (due to not needing to recharge gas cylinders so frequently).